Make vec_slice() size-aware

[jameslairdsmith]

An idea for making vec_slice() aware of the size of the vector. One way to do this would be similar to what happens in dplyr, where there is n(), which in this case would only work inside the call to vec_slice().

library(vctrs)

vec_slice_n <- function(x, i, ...) {                                          
  i_quo <- substitute(i)                                                      
  size <- vctrs::vec_size(x)                                                  
  mask <- new.env(parent = parent.frame())                                    
  mask$n <- function() size   
  idx <- eval(i_quo, envir = mask)                                            
  vctrs::vec_slice(x, idx, ...)                                               
}

vec_slice_n(letters, n())            
#> [1] "z"
vec_slice_n(letters, c(1, n()))      
#> [1] "a" "z"
vec_slice_n(letters, (n() - 2):n())  
#> [1] "x" "y" "z"
vec_slice_n(letters, -n())  
#>  [1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s"
#> [20] "t" "u" "v" "w" "x" "y"

 `vec_slice_n<-` <- function(x, i, ..., value) {
   i_quo <- substitute(i)
   size <- vctrs::vec_size(x) 
   mask <- new.env(parent = parent.frame())
   mask$n <- function() size
   idx <- eval(i_quo, envir = mask)
   vctrs::`vec_slice<-`(x, idx, ..., value = value)
}

y <- 1:5; vec_slice_n(y, n()) <- 99; y            
#> [1]  1  2  3  4 99
z <- 1:5; vec_slice_n(z, c(1, n())) <- c(-1,-9); z 
#> [1] -1  2  3  4 -9
w <- 1:5; k <- 2; vec_slice_n(w, (n()-k):n()) <- 0; w
#> [1] 1 2 0 0 0

^{Created on 2026-06-30 with reprex v2.1.1}

[DavisVaughan]

Hmm, I don't think we want to add these special context aware semantics to a low level package like vctrs. I also think this is fairly easy to do to get the same kind of behavior

vec_slice(letters, vec_size(letters))
vec_slice(letters, -vec_size(letters))